The invention relates to a method and an apparatus for turning off a cascode circuit comprising a series circuit formed by a low-blocking-capability and high-blocking-capability semiconductor switch.
In power electronic circuits, because of internal switching actions or external mains overvoltages, voltage values can occur across power semiconductors which exceed their rated blocking capability. Since the occurrence of such operating states cannot be precluded, an elementary requirement for protecting power semiconductor elements is the ability to withstand overvoltages, in order to avoid destruction of the semiconductor components or more extensive damage.
For this problem, two possible solutions currently exist, namely passive and active overvoltage limiting.
Passive overvoltage limiting employs a capacitor which is electrically connected in parallel with the power semiconductor. This protective circuit is also referred to as a clamp circuit. By means of this clamp circuit, the energy of the overvoltage is conducted into the capacitor, thereby limiting a voltage rise at the power semiconductor switch. This capacitor may additionally be augmented by a resistor and a diode to form a so-called RCD protective circuit, which is also referred to as a snubber circuit. The disadvantage of the clamp protective circuit or of the snubber circuit is that the capacitor has to be designed for the maximum voltage that occurs. Such a capacitor is costly and occupies a comparatively large structural volume. Furthermore, the leakage inductance of the protective circuit will increase because of the large structural volume and the longer connection lines resulting therefrom. If no further outlay is to be expended, the energy stored in the capacitor is converted into heat by means of the protective circuit resistor, as a result of which this positive overvoltage protection causes a non-negligible power loss.
In the case of active overvoltage protection, an overvoltage that occurs at the power semiconductor is detected and compared with a limit value which is less than a maximum reverse voltage of the power semiconductor, and the power semiconductor is actively driven as soon as the overvoltage exceeds the predetermined limit value. As a result of the active driving, the power semiconductor is able to convert the energy of the overvoltage into heat by an elevated current flowing through the power semiconductor at high voltage. An overvoltage can be identified by a voltage-limiting component, for example a zener diode, which carries a current in the reverse direction when its zener voltage is exceeded. This current can be passed directly or via an amplifier into the control terminal of the power semiconductor in order to turn the latter on. In this arrangement, a high-voltage zener diode is required. Instead of a high-blocking-capability zener diode, it is also possible to use a high-impedence voltage divider for identifying the overvoltage. The disadvantage of the active overvoltage protection described is that the detection elements have to be designed for the entire reverse voltage of the power semiconductor. Moreover, a high-impedence voltage divider continually causes a power loss, whereas a high-blocking-capability zener diode is thermally endangered by the power loss converted in it. Moreover, high-blocking-capability components are expensive.
German patent specification 196 10 135 C1 discloses a cascode circuit of two voltage-controlled semiconductor switches which are electrically connected in series. This cascode circuit is described below with reference to FIG. 1 of the drawings.
In FIG. 1, cascode circuit 2 has a low-blocking-capability semiconductor switch 4 and a high-blocking-capability semiconductor switch 6, which are electrically connected in series. A normally off n-channel MOSFET, in particular a low-voltage power MOSFET, is provided as the low-blocking-capability semiconductor switch 4, and an n-channel junction FET is provided as the high-blocking-capability semiconductor switch 6. This high-blocking-capability junction FET 6 is also referred to as Junction Field-Effect Transistor (JFET). The two FETs 4 and 6 are electrically connected in series in such a way that the source terminal of the JFET 6 is electrically conductively connected directly to the drain terminal Dxe2x80x2 of the MOSFET 4 and the gate terminal of the JFET 6 is electrically conductively connected to the source terminal S of the MOSFET 4 by means of a gate resistor RGJ. This electrical interconnection of two semiconductor components is referred to as a cascode circuit, as is known. Since respective FETs are used as the semiconductor switches 4 and 6 of the cascode circuit 2, this cascode circuit 2 is also referred to as a hybrid power MOSFET. The low-blocking-capability MOSFET 4 of this cascode circuit 2 has an internal biplar diode DIN, which is reverse-connected in parallel with the MOSFET 4 and is generally referred to as an inverse diode or internal freewheeling diode. The normally off n-channel MOSFET 4 is made of silicon, whereas the normally off n-channel JFET 6 is preferably composed of silicon carbide. This hybrid power MOSFET 2 is designed for a high reverse voltage of more than 1000 V and nevertheless has only small losses in the on-state range.
FIGS. 2 and 3 illustrate blocking characteristic curves of the normally on n-channel JFET 6 and of the normally off n-channel MOSFET 4, respectively, in a plot of reverse current against reverse voltage. Since the low-blocking-capability and high-blocking-capability semiconductor switches 4 and 6 are electrically connected in series in the cascode circuit 2, the current through both semiconductor switches 4 and 6 must be of the same magnitude. Moreover, the reverse voltage UDSA of the low-blocking-capability semiconductor switch 4 is present as gate voltage at the high-blocking-capability semiconductor switch 6 of the cascode circuit 2. If a reverse voltage UDSA is then present at the turned-off cascode circuit 2, it will be divided between the two semiconductor circuits 4 and 6 of the cascode circuit 2. This division will be effected such that the same reverse current IDA is established for both semiconductor switches. A stable operating point AP will be established in this way.
If, from a stable operating point AP of the cascode circuit 2, the value of the reverse voltage UDxe2x80x2SA of the low-blocking-capability semiconductor switch 4 should shift to low values, then the reverse current IDA would likewise have to decrease in accordance with the blocking characteristic curve according to FIG. 3. For the high-blocking-capability semiconductor switch 6, this means only a marginal change in its reverse voltage UDSA, since the latter is significantly greater than the reverse voltage UDxe2x80x2SA of the low-blocking-capability semiconductor switch 4. A decrease in the reverse voltage UDxe2x80x2SA of the low-blocking-capability semiconductor switch 4 of the cascode circuit 2 likewise means a decrease in the magnitude of the gate voltage of the high-blocking-capability semiconductor switch 6. A reduced-magnitude gate voltage of the high-blocking-capability semiconductor switch 6 means an increased reverse current IDA (FIG. 2). However, this increased reverse current IDA can only be carried by the low-blocking-capability semiconductor switch 4 of the cascode circuit 2 if said switch takes up a larger reverse voltage. Consequently, the previously conceived decrease in the reverse voltage UDxe2x80x2SA of the low-blocking-capability semiconductor switch 4 of the cascode circuit 2 is cancelled.
This fact can be utilized for overvoltage identification, a low-voltage signal being used to detect an overvoltage at high potential.
The invention has as its object, the ability to detect an overvoltage at high potential by means of a low-voltage signal.
By virtue of the fact that when a switch-off signal arrives, the gate voltage of the low-blocking-capability semiconductor switch is controlled in such a way that its drain voltage is held constant in an active range of the low-blocking-capability semiconductor switch of the cascode circuit, a low-voltage signal is obtained which indicates whether an overvoltage occurs at high potential of the cascode circuit.
Since drain voltage of the low-blocking-capability semiconductor switch of the cascode circuit is held constant in the active range, only the drain current through the cascode circuit can rise when an overvoltage occurs, owing to the constant gate voltage. This current rise can only be carried by the low-blocking-capability semiconductor switch of the cascode circuit if, at constant drain voltage, the gate voltage is altered in such a way that the low-blocking-capability semiconductor switch is turned on again. In other words, the direction of the gate voltage change is changed. This change in the gate voltage is tapped off at low potential and is an indication of the occurrence of an overvoltage at high potential of the cascode circuit.
In addition to a predetermined desired value, the actual value of the drain voltage is also required for controlling the drain voltage of the low-blocking-capability semiconductor switch of the cascode circuit and this required low-voltage signal can also be evaluated with regard to overvoltage detection. In the normal case, the value of this drain voltage actual value that has been determined corresponds approximately to the value of the drain voltage desired value. If an overvoltage occurs during switch-off or in the off state of the cascode circuit, then the actual value of the drain voltage of the low-blocking-capability semiconductor switch of the cascode circuit rises rapidly. This rise in the actual value of the drain voltage is then an indication of the overvoltage at high potential of the cascode circuit.
The apparatus for turning off a cascode circuit, according to the invention, comprises a control loop, including a comparator and a controller, which, when the cascode circuit is turned off and optionally also in the switched-off state, is linked to the gate terminal of the low-blocking-capability semiconductor switch of said cascode circuit. At low potential, the control loop offers a plurality of signals which can be evaluated for overvoltage detection. The signal which is offered for overvoltage detection depends on the configuration of the controller or on the dynamic range of the control loop. A sign change in the drain voltage difference value is an indication of the occurrence of an overvoltage at high potential of the cascode circuit. Alternatively, the rise in the actual value of the drain voltage of the low-blocking-capability semiconductor switch can be used as a detector signal with regard to the occurrence of an overvoltage at high potential of the cascode circuit.
With the method according to the invention for turning off a cascode circuit, the occurrence of an overvoltage at high potential of the cascode circuit can be detected with the aid of a low-potential signal, the cascode circuit converting the energy of the overvoltage into heat. At the end of the switch-off operation, the high-blocking-capability semiconductor switch and the low-blocking-capability semiconductor switch are in the off state. In the off state, the drain voltage of the low-blocking-capability semiconductor switch is either controlled to a predetermined constant value by way of its gate voltage or is set to an arbitrary value with the gate turned off.
Consequently, it is possible not only to detect an overvoltage at high potential by means of the method according to the invention for turning off a cascode circuit, but simultaneously to actively limit this overvoltage.